1. Field of the Invention
The present invention relates to an exhaust gas purification device for an engine.
2. Related Art
In a known an exhaust gas purification device for a diesel engine, an oxidation catalyst for oxidizing nitrogen monoxide NO in the exhaust gas into nitrogen dioxide NO2 is arranged in the exhaust passage and a particulate filter for trapping particulates in the exhaust gas is arranged in the exhaust passage downstream of the oxidation catalyst (see Japanese Unexamined Patent Publication No. 2001-318715). Specifically, the air-fuel ratio of the exhaust gas flowing into the particulate filter is maintained lean and, therefore, when the temperature of the particulate filter becomes relatively high, particulates, mainly composed of solid carbon, deposited on the particulate filter are oxidized and removed. In this case, if NO2 is contained in the exhaust gas flowing into the particulate filter, it promotes oxidation of particulates and reduces the temperature, of the particulate filter, required for oxidizing particulates. In the exhaust gas purification device described above, therefore, the oxidation catalyst is arranged upstream of the particulate filter to convert NO in the exhaust gas into NO2.
An amount of NO2 flowing into the particulate filter, however, depends on an amount of NO discharged from the combustion chamber and may vary in accordance with the engine operating condition. Accordingly, if a condition in which the amount of NO discharged from the combustion chamber is small, and is continued for a long time, oxidation of particulates fails to be promoted and, therefore, the particulate filter may be clogged.